West Nile virus can be deadly, but the vast majority of people who get infected never become seriously ill. About 80% of infections cause no symptoms at all. Of those who do get sick, most experience a mild fever that resolves on its own. The real danger comes from a rare but serious form of the disease that attacks the brain and spinal cord, which kills roughly 9% of the people it affects.
Most Infections Go Unnoticed
West Nile virus spreads through mosquito bites, and the odds are heavily in your favor if you’re exposed. For every 100 people infected, about 80 will never know it happened. They won’t develop a single symptom. The remaining 20 or so will come down with what’s called West Nile fever: headache, body aches, fatigue, and sometimes a skin rash. Symptoms typically show up 2 to 6 days after a mosquito bite, though it can take up to two weeks.
West Nile fever is unpleasant but rarely dangerous. Fewer than 1% of nonneuroinvasive cases are fatal. Most people recover completely, though lingering fatigue and weakness can drag on for weeks or even months.
When the Virus Reaches the Brain
In less than 1% of all infections, the virus crosses into the central nervous system. This is called neuroinvasive disease, and it’s where West Nile becomes genuinely dangerous. It can cause inflammation of the brain (encephalitis), inflammation of the membranes surrounding the brain and spinal cord (meningitis), or a polio-like paralysis that strikes suddenly.
CDC surveillance data from 2009 to 2018 found that 9% of reported neuroinvasive cases were fatal. That means roughly 1 in 11 people who develop the severe neurological form of the disease die from it. Recovery for survivors is slow, often taking weeks to months, and some people are left with lasting neurological problems.
In 2025, the United States reported 2,076 total human cases across 47 states, with 1,434 of those classified as neuroinvasive. That high proportion of severe cases in the reported numbers reflects an important reality: mild cases are massively underreported because most people with a brief fever never get tested. The true number of infections each year is far higher than what shows up in official counts.
Who Faces the Greatest Risk
Age is the single biggest risk factor for severe disease. Older adults, particularly those over 60, are significantly more likely to develop the neuroinvasive form and to die from it. The risk also climbs with certain chronic conditions: diabetes, high blood pressure, cancer, and kidney disease all increase vulnerability. People with weakened immune systems, whether from a medical condition or from medications that suppress immune function, face elevated risk as well.
Healthy younger adults and children can develop severe disease, but it’s uncommon. When they do get infected, the overwhelming likelihood is a mild illness or no symptoms at all.
No Vaccine, No Specific Treatment
There is currently no vaccine for West Nile virus in humans and no antiviral medication that targets it. Treatment for severe cases is supportive: hospital care focused on managing symptoms like brain swelling, breathing difficulties, and dehydration. For mild cases, rest, fluids, and over-the-counter pain relievers are the standard approach.
Diagnosis typically involves a blood test looking for antibodies the body produces in response to the virus. These antibodies usually become detectable 3 to 8 days after symptoms start. In suspected neuroinvasive cases, spinal fluid may also be tested.
Prevention Comes Down to Mosquitoes
Since there’s no vaccine or cure, avoiding mosquito bites is the only real line of defense. The CDC recommends using EPA-registered insect repellents containing DEET, picaridin, IR3535, or oil of lemon eucalyptus. These have been evaluated for both safety and effectiveness, including for pregnant and breastfeeding women when used as directed. Natural repellents that aren’t EPA-registered have no proven track record against disease-carrying mosquitoes.
Beyond repellent, practical steps make a difference: wearing long sleeves and pants during peak mosquito hours (dawn and dusk), using window screens, and eliminating standing water around your home where mosquitoes breed. Even a bottle cap’s worth of stagnant water can serve as a breeding site.
Putting the Risk in Perspective
West Nile virus is the leading cause of mosquito-borne disease in the continental United States, and it circulates every summer. But the math is reassuring for most people. If you’re bitten by an infected mosquito, there’s an 80% chance nothing happens, a roughly 19% chance of a mild fever, and less than a 1% chance of the severe neurological disease that carries real danger. That small percentage, though, translates to hundreds of serious cases and dozens of deaths in the U.S. every year, concentrated among older adults and people with compromised health.